In recent years, electronic devices that are equipped with an organic EL (Electro Luminescence) display panel and that perform an operation when an operator touches a screen displayed thereon with a finger, a pen, or the like have been spreading widely. In the display panel of such an electronic device, photo-detection sections that detect an amount of light reflected by a finger or the like are disposed in a matrix so as to detect a touch position of a finger, a pen, or the like. The electronic device detects a touch position on the screen based on the amount of light detected by the photo-detection sections disposed in the display panel, and conducts an operation desired by an operator.
In relation to this, the WO 2007/145347 pamphlet discloses a liquid crystal display panel that includes photo-detection sections each having a photodiode and disposed for each pixel. The photo-detection section includes a photodiode that generates electric charges corresponding to incident light, a capacitor that accumulates the electric charges generated in the photodiode, and a TFT (Thin Film Transistor) that acts as an output amplifier. The operation of the photo-detection section will be later described in detail, but when light reflected by a finger or the like is incident on the photodiode, electric charges corresponding to the light amount are generated in the photodiode, and the generated electric charges are accumulated in the capacitor, thereby lowering a voltage retained by the capacitor by an amount corresponding to the amount of the accumulated electric charges. When the voltage retained by the capacitor is applied to a gate terminal of the TFT, the TFT outputs a voltage that is lower than the voltage retained by the capacitor by the threshold voltage. In this way, the photo-detection section outputs a voltage corresponding to the light amount, and therefore, the liquid crystal display device can obtain the coordinate of a touch position on the screen based on the output voltage of the photo-detection section.